Among the British writers who've made names for themselves in the last 15 years or so, Ali Smith stands out as one of the most original, or at least the hardest to categorise. Smith has served time in the academic world, and the more self-conscious side to her writing sometimes attracts the label "experimental". But most aspects of her output, from the everyday diction to the ragged right-hand margins she's gone in for since Hotel World (2001), give off an air of reader-friendly homemadeness rather than lab-coated authority. She alludes freely to other writers but has never seemed dominated by any particular influence; although politics is everywhere in her work, it's elusive too. And while she's interested in style, in finding voices for her characters and using language in unexpected or counter-conventional ways, she's not interested in stylishness as John Updike, for example, would understand it. If an occasion calls for flat or wilted writing, that's fine by her.

Smith is also unusual among British writers in being committed to the short story, a form whose health in this country tends to give rise to either gloom or embattled boosterism. The First Person, her fourth and latest collection, opens with a piece called "True Short Story" that was apparently written "in playful response" to a speech by Alex Linklater, one of Smith's fellow boosters. In it, the narrator, Ali, overhears a young man in a cafe comparing the novel to "a flabby old whore" and the story to a nymph who's in much better shape on account of having been "mastered" by fewer people. Ali's first response to this sniggering talk isn't prim: she wonders "how many of the books in my house were fuckable and how good they'd be in bed". Then she rings her friend Kasia, a breast cancer sufferer and authority on short stories. How is a story like a nymph? After some banter, a flashback and a rewriting of Ovid, the two women arrive at a more acceptable answer.

On one level, "True Short Story" is a straightforward celebration of women finding their voices. Ali takes over the young man's comparison; her version of the nymph Echo is more mutinous than Ovid's; we see Kasia speaking back to both a snooty Cambridge don and an NHS funding body. But the reader is also invited to consider thoughts on the short story form by luminaries ranging from Grace Paley to Ernest Hemingway. "Alice Munro," Smith writes, "says that every short story is at least two short stories," and that's certainly true of this one: there's an inward-looking story concerned with finding analogies for stories themselves, and an outward-looking one concerned with a campaign to have Herceptin made available on the NHS. By putting the characters who stand in for the real-life Ali Smith and her friend Kasia Boddy into a fictional frame, Smith also points up her title's ambiguity: does it mean a story that's strictly factual, one that achieves a less literal kind of truth, or one that's true to the nature of the form?

The next piece, "The Child", is very different. "I went to Waitrose as usual in my lunchbreak," it flatly begins, "to get the weekly stuff." The narrator leaves her trolley for a moment, and when she returns her Kalamata olives and copy of the Guardian have been joined in it by a baby. This baby - which no one claims and which everyone insists is obviously hers - is incredibly beautiful, and the narrator ends up driving off with it. Then, in an endearingly lisping RP accent, the baby begins to speak: "The pound is our rightful heritage. We deserve our heritage. Women shouldn't work if they're going to have babies ... And as for gay weddings. Don't make me laugh."

At first, the narrator is charmed against her will, and the story ends in a way that leaves open the question of whether the baby is an emanation of conservative notions about motherhood, of a parent's increased receptiveness to right-wing appeals, or of the tabloid newspapers highlighted in the closing paragraph.

All that's needed by now for contrast is one of the no-tricks, almost anecdotal pieces that Smith showed she can write in her first collection, Free Love (1995). As the stories pile up, though, it becomes increasingly clear that these aren't on the menu: metafictional gestures, artful narrative framing and bold imaginative conceits are the order of the day. "Fidelio and Bess" uses Gershwin and Beethoven to break open a cleverly dramatised love affair. In "Writ", an unexpected kiss causes the middle-aged narrator's 14-year-old self to materialise in her living room. There's also a group of related stories - "The Third Person", "The Second Person", "The First Person" - exploring the dynamics of the corresponding grammatical persons. Words addressed to the characters blur into praise for the relevant personal pronouns: "You're something else, you. You really are."

These technical ploys are all carried off impressively, and Smith cracks some good jokes along the way. But after 200 pages, a few weaknesses emerge as well. While her supply of narrative hooks appears to be inexhaustible, she rarely develops them dramatically: "The Third Person", for example, is oddly static beneath its busy surface. The numerous "you" and "I" characters aren't strongly particularised, coming across as variations on the same lovers or ex-lovers, and the lyrical mode of address occasionally falls into grandiosity or preciousness: the prose starts rhyming ("a shut-eye lie"; "doomed on sea, you and me") or reaching for show-stoppingly self-referential metaphors ("if there's a library anywhere near then someone just removed its roof, the shelves just flooded with sun and all the old books just remembered what it means to be bound in skin and to have a spine"). Most of all, you sometimes get the feeling that this talented writer could knock out similar stories in her sleep, each one lively and inventive but dreamily absorbed in the protocols of its own making.